This invention relates to stable zeolite/cellulose composite materials and, more particularly, to a general, facile method for preparing such composite materials.
Synthetic and natural zeolites, a family of aluminosilicates with pores and cavities in the range 4-18 xc3x85, are well known heterogeneous catalysts and absorbents. The preparation of a soft membrane with molecular sieving properties is attractive from both practical and fundamental standpoints. One general approach is to combine a crystalline, yet porous solid such as zeolites, with a flexible, cheap and abundant organic matrix, such as natural cellulose [S. Mintova, V. Valtchev, Zeolites 16 (1996), 31]. Once a stable zeolite/cellulose material is synthesized, carton- or paper-like zeolite/cellulose filters can be produced by contacting the composite with distilled water to incipient wetness, followed by applying pressure to 2-5 ton/cm2 at 383 K for a few minutes. A porous zeolite/cellulose filter paper manufactured this way will have molecular sieving properties, and as such, having some degree of perm-selectivity makes it a membrane by definition.
Zeolite/cellulose and cellulose acetate composites have already been proposed for numerous uses. For example, they have been tested for medical applications (medical antibacterial materials, denture plaque control (H. Nikawa et al., J. of Oral Rehabilitation 24 (1997) 350]), fragrance products (tissue conditioner [H. Nikawa et al. supra]), fabric softeners (T. McGee et al. EP Pat. 1,061,124, 2000) deodorizers (removal of NH3, H2S [H. Haruhito et al., JP Pat. 63,224,734 (1988)] and EtNH2 [Y. Takashi et al., JP Pat. 86,241,059 (1988)], and filtrations (ultrafiltration membrane [Z. Mara et al., J. Membr Sci. 36 (1988) 277], dewatering sludges [Y. Yoshiya et al., JP Pat. 84,158,863 (1986)], tobacco smoke filters [Keith, U.S. Pat. No. 3,327,718 (1967)], water purification [Katsumi et al., JP Pat. 92,286,299 (1994)], air purification [Norimitsu et al., JP Pat. 92,129,440 (1993)], filtration membranes for aerated water and wine [Kazusumi et al., JP Pat. 91,177,260 (1991)], and adsorbent filter for decaffeinating processes [Cohen et al., U.S. Pat. No. 5,906,743 (1999), Everhart et al., U.S. Pat. No. 5,603,830 (1997) and Everhart et al., U.S. Pat. No. 5,728,634 (1998)]. Other applications related to adsorption process have also been proposed. Among these are absorbent pads (antimicrobial water-absorbing sheet for food [Junichi et al., JP Pat. 110,032,748 (1999)], adsorbent sheets [Japan Vilene Co., JP Pat. 55,160,035 (1980)], disposable diapers, sanitary napkins, panty shields, underarm shields and incontinence pads [Yeo et al., U.S. Pat. No. 5,122,407 (1992)], gas and liquid separations (CO2/CH4, O2/N2 [Duval et al., J. Membr. Sci. 80 (1993), 189], olefin/paraffin [Huang et al., Prepr, ACS Div. Petr. Chem. 46 (2001), 166], monosaccharide/polysaccharide [Kulprathipanja et al., U.S. Pat. No. 4,735,193 (1985)], and refining processes (decoloration and decalcifying of sugar syrup [Kunin et al., U.S. Pat. No. 4,572,742 (1986)].
The prior art is that zeolite/cellulose composite materials can be prepared by using adhesive polymers [Korchakov et al., RU Pat. 2,109,767 (1988, Katsunao JP Pat. 2,000,189,793 (2000), M. Fujio, Hyoman Kagaku 19 (1998), 658, Sano et al., Proc. Int. Zeolite Conf. 9th (1993), 239-246) and Toshio et al. JP Pat. 63,035,886 (1998)], electret technology [Cohen et al. U.S. Pat. No. 5,906,743 (1999)], or in situ zeolite crystallization [Mintova, Valtchev, Zeolites 16 (1996), 31, Mintova, Valtchev, Preparation of Zeolite-Covered Cellulose Fibers, 209th National Meeting Abstracts, American Chemical Society (1995), and Niu, xe2x80x9cA Study on the Preparation of Zeolite Y-Cellulose Composite Materials, M.S. thesis (2000), University of Nebraska-Lincoln]. As to the last strategy, it has been noted that untreated cellulose fibers have a marked tendency, at different temperatures and Si/Al starting ratios, to induce crystallization of zeolite 5A particles on their surface [Niu, supra]. The sparse work reported in the open literature on in situ zeolite crystallization over cellulose revolves around 5A/cellulose composites. (See Mintova and Valtchev supra).
The adhesive polymer method was used in industry for several decades. Mintova and Valtchev (supra) indicated that there were several disadvantages associated with this process, the most important ones being the weak zeolite/cellulose interaction, and the uneven distribution of zeolite crystallites on the cellulose surface. In addition, incorporating an adhesive (a third chemical entity) in the zeolite/cellulose composite might be deemed unacceptable for certain applications, especially if the polymer is unstable or soluble in the filtration medium. The so-called xe2x80x9celectretxe2x80x9d method requires the use of high voltages, which might damage substances and poses an occupational risk. The third synthetic approach normally yields by-products and materials that are unstable on washing with boiling water. (Niu, supra).
There remains a need for a method of preparing zeolite/cellulose composite materials which is easy to carry out and which produces composite materials stable upon prolonged boiling water washing.
Among the several objects of the invention may be noted the provision of a method for preparing stable cellulosic fiber material for use in forming zeolite/cellulose composites; the provision of such a method which may be readily carried out in a cost-effective manner; the provision of novel zeolite/cellulose composite materials in which the zeolite content is characterized by being stable on washing with water at 373 K for several hours; and the provision of zeolite/cellulose composite materials produced by a method which makes use of commercial zeolite powders and cellulose sources. Other objects and features will be in part apparent and in part pointed out hereinafter.
Briefly, the present invention is directed to the method of preparing stable cellulosic fiber material for use in forming zeolite/cellulose composites comprising the steps of (a) suspending loose cellulose fibers in an aqueous solution of sodium hydroxide, potassium hydroxide or sodium silicate; (b) stirring the resulting suspension until it reaches a macroscopically homogeneous appearance; (c) heating the resulting mixture at a temperature of approximately 323-423 K until only dry solids remain; (d) contacting the resulting mixture with excess distilled water to remove physically adsorbed or trapped compound of step (a) from the fibers; and (e) heating the resulting fiber material at a temperature of approximately 323-423 K to dry the fiber material. The invention is further directed to a stable zeolite/cellulose composite material characterized in that leaching of the zeolite phase of the composite does not occur upon contact of the composite with water at approximately 373 K, the composite material comprising a zeolite and a stable cellulosic fiber material prepared by the above-described method. The invention is also directed to the method of preparing a stable zeolite/cellulose composite material comprising the steps of preparing a stable cellulosic fiber material by the above-described method, contacting a zeolite with the stable cellulosic fiber material; stirring the resulting solid-liquid suspension until macroscopic homogeneity is achieve, heating the resulting composite material at a temperature of approximately 323-423 K, washing the resulting composite material, and drying the resulting composite material.